On Fri, 14 Apr 2006 10:09:37 -0500, Joseph Gwinn
wrote:

In article ,
Eric R Snow wrote:

On Tue, 11 Apr 2006 23:24:21 -0500, Joseph Gwinn
wrote:

In article ,
Eric R Snow wrote:

[snip]
Years ago I was blowing out some bearings with air. Goofing off, with
the bearing on my finger, I spun one up and listened to the pitch. As
it spun the pitch got higher until I couldn't hear it. Just as it
passed into my ultrasonic range it exploded with a bang.

Ultrasonic! Wow. That implies a ball-passing frequency exceeding
20,000 per second or so. If the ball cage has ten balls in it, the cage
is rotating at 2000 rps, or 120,000 rpm, and the outer race a factor
faster. No wonder it exploded.

MY ultrasonic range. I know my high frequency hearing is damaged. But
it was still really spinning fast.

How old were you when this bearing died?

In my 20s and 30s, I could hear to 20 KHz, and sense 26 KHz (used by
some burglar alarms) as a pressure.


Do you recall the dimensions of the deceased ball bearing?
The ID was about 5/8 and the OD maybe 1-1/2, 1-3/4.

OK. Not that large.


The bearing
axis was perpendicular to my body so that the bearing parts were
embedded into the wall and not me. My finger hurt like hell. I think
the bearing must have exploded pretty equally because otherwise that
finger would have probably broken instead.

Yes. Lab ultra-centrifuges are built inside a heavy steel box, to
contain the shards of the rotor, when (not if) it explodes. When this
happens, the centrifuge is totally destroyed.


I don't spin up bearings
any more. Not even a little.

It sounds like there is actually quite a wide safe range here. Just
stay in the sonic range?

No way. A small bearing may take the rpm. But a larger one will
explode much sooner. Just don't spin 'em up.

The tone indicates rotation rate. The larger the outer diameter, the
greater the centrifugal force on the outer race. The effect is very
much like a gravitational field pulling outward, and can easily be tens
of thousands of times stronger than Earth's gravity.

Making the race heavier doesn't help, because while the cross sectional
area (and thus strength) increases, so does the weight (and thus outward
force). My recollection from discussions of energy-storage flywheels is
that these effects cancel more or less perfectly, so all that matters is
the strength-to-weight ratio of the material from which the outer race
is made. Only hoop stress matters, so storage flywheels are often made
of boron fiber.

What alloy are bearing races made of? What's the tensile strength?

Joe Gwinn
Greetings Joe,
I was in my early twenties when I blew up the bearing. However, I
already had some hearing loss by then. When I was about 12 I could
hear all sorts of high frequency sound that others around me couldn't.
Used to drive me nuts. High pitch coming out of the TV. And my grandma
had a remote control that used sound (remember those?) to switch
channels on the TV that bugged me too. I couldn't really hear it, I
mean it didn't sound like "sound", but when the hammer in the remote
struck the little metal bar inside it was sorta like the pressure you
wrote about.
ERS